Ethanol boiling point and vapour strength calculator
Turn a temperature into a strength, or a strength into a temperature: the strength of the liquid in the boiler and of the vapour above it, from a thermometer reading - corrected for your elevation or air pressure.
The background is in the temperature chart and distilling at altitude.
How to use it
- Say what you know. A boiler temperature, a head temperature, or the strength of the liquid in the boiler.
- Enter the reading. In °C or °F, from a calibrated thermometer whose sensor sits in the liquid (boiler) or in the vapour just below the outlet (head).
- Set your air pressure. Enter your elevation for a typical figure, or the actual pressure from a barometer - not the sea-level pressure in a weather report.
- Read the strengths. The chart shows where your reading sits on the boiling curve (liquid) and the vapour curve.
How it works
A boiling mixture of alcohol and water gives off a vapour richer in alcohol than the liquid it comes from, and the stronger the liquid, the cooler it boils. Each boiling temperature therefore matches one liquid strength and one vapour strength.
The calculator finds that pair from the vapour pressures of ethanol and water (the NIST Antoine equations) and a standard model of how the two affect each other in a mixture. Its results agree with published measurements to within about half a degree. Because it is built on vapour pressures, it works at any air pressure: higher up, everything boils cooler, but the strengths are the same.
A thermometer in the head reads the vapour's temperature only once the still has settled and the vapour is passing the sensor. That is why this tells you about your still, but can't tell you when to make the cuts: heads and tails are judged by smell and taste.
Worked examples
At sea level, a boiler at 92 °C holds liquid of about 11.6% ABV, giving off vapour of about 57.7%.
At 1,500 m, the same 92 °C means something quite different: liquid of only 3.6% and vapour of 30.5% - the run is nearly over. A 10% wash, which boils at 92.8 °C at sea level, boils at 88.0 °C there.
Where Stilligent fits
Stilligent does these sums for you, live. From its boiler and column-head sensors it shows a running estimate of the strength in the boiler and at the head throughout the run, and charts the temperatures. The air pressure comes from its own pressure sensor - or from an online weather service, a reading entered by hand, or simply your altitude.
It uses the same pressure, and the strength of your wash, to suggest each cut, from foreshots to tails, as the run reaches it.
You stay in charge: one tap makes a cut whenever your nose and palate say so, and Not yet keeps collecting. It watches the column-head sensor closely, too: if it is unplugged, stops reading or drops suddenly, Stilligent cuts power to the boiler and sounds the alarm, to help prevent an accident.
Questions
Can I make the cuts by temperature?
No. Temperature tells you roughly how strong the liquid and vapour are, but not which compounds are coming over. Cuts are made by smell and taste - the making the cuts guide shows how.
Why does the head of a reflux still sit at about 78 °C for so long?
A good reflux column delivers vapour close to the strongest possible mixture, which boils at about 78 °C at sea level, so the head stays there until the boiler runs low on alcohol.
Why can't I distil above about 96% ABV?
At around 95–96% ABV ethanol and water boil together as if they were one liquid (an azeotrope), so the vapour is no stronger than the liquid and distillation can go no further.
Does altitude change the strength I get?
No - only the temperatures. Every strength boils cooler higher up, by roughly 1 °C for every 300 m. The altitude guide explains.
Why does my boiler read hotter than my head?
Heat is lost on the way up, and in a reflux still the vapour is enriched as it rises, so the head is cooler. In a simple pot still the two are close.
More calculators for distilling
Free to use, in metric or US units; your settings are remembered on this device. All results are estimates for planning: check strengths with a hydrometer or alcoholmeter. Distil only where it is legal to do so, and never leave a running still unattended.
Underlined terms are explained in the glossary - tap one for a quick definition.